1. Technical Field of the Invention
The present invention is related to wireless telecommunications networks. In particular, the present invention relates to cellular telephone networks. More particularly, the present invention relates to techniques for analyzing downlink interference in a wireless telecommunications network. Still more particularly, the present invention relates to techniques for identifying interfering sources in wireless telecommunications networks.
2. Description of the Related Art
Present-day mobile telephony has spurred rapid technological advances in both wireless and non-wireless areas. The telecommunications industry is a rapidly growing industry, with advances, improvements, and technological breakthroughs occurring on an almost daily basis. Many mobile telecommunications systems, among them the European GSM-system, have already passed through several basic development phases and system designers are now concentrating on further improvements to the systems, including system refinements and the introduction of optional services.
Most wireless telecommunication systems currently in use are implemented as cellular telephone networks. Cellular telephone networks typically are composed of a group of base stations connected to a centrally located switch. This centrally located switch is commonly referred to as a Mobile Switching Center (MSC). Base stations are usually spaced apart from each other by distances of between one-half and twenty kilometers. Each base station is assigned a number of two-way voice and control channels. The voice channels transmit voice signals to and from proximately located mobile stations, and transmit control information to and from these mobile stations, usually for the purpose of establishing a voice communications link.
A typical cellular telephone network also includes a group of interconnected mobile switching centers, which operate in association with a gateway mobile switching center through which the cellular telephone network interconnects with a conventional public switched telephone network (PSTN). In addition, at least one home location register (HLR) operates within the cellular telephone network. The HLR stores network subscriber information, including the identified location of current mobile stations within the network.
In response to an incoming call dialed to a mobile station, a signal is sent to the home location register requesting routing information through the network to the called mobile station. The home location register "looks up" the current location of the mobile station and contacts the currently serving mobile switching center to pre-route the call and retrieve a temporary location directory number, which is utilized to route the call through the cellular telephone network for delivery to the mobile station. The serving mobile switching center retrieves from a visitor location register (VLR), the identification of the cell within which the called mobile station is currently located. The mobile switching center then instructs the base station associated with that particular cell to page the mobile station. Responding to the page, the mobile station requests assignment of a channel, and the network routes the call through the serving mobile switching center and over the assigned channel.
It is important to identify those cells within the cellular telephone network, which are sources (offenders) of and subject (victims) to radio disturbances. A radio disturbance event typically occurs during a cellular call, either on the downlink from a base station to a mobile station (e.g., a cellular telephone), or during an uplink. A radio disturbance occurs due to several possible reasons, including co-channel interference, adjacent channel interference or external interference. Various methods exist for identifying a disturbed call. In order to identify cells that are disturbed (i.e., "victims" of disturbance), a comparison of signal strength versus a measurement of speech quality can be employed (i.e., bit error rate). When sufficient signal strength is correlated with degraded speech quality for an extended period of time, usually measured in seconds, that cell can be considered "disturbed." The length of time is dependent on the technology available and the measurement method utilized. Failure to identify and analyze sources of radio disturbances and the disturbed cells in the cellular telephone network results in poor communication customer dissatisfaction.
Various methods and systems currently exist for identifying disturbed cells within cellular telephone networks. One of the most widely utilized methods involves downlink interference prediction tools. Such tools predict where interference may exist within a given cellular telephone network. The predictions are then utilized for frequency and cell planning, particularly in initial network designs. The validity of such predictions is dependent on a number of factors, including the accuracy of the propagation model utilized, the resolution of the terrain data, and so forth. Such tools are helpful in identifying the cells that are causing the downlink interference, but taken together are often inaccurate because of the dependence on predictions.
Another method utilized to identify disturbed and offending cells involves drive testing. The drive test approach includes method steps of turning off all co-channel transmitters within a particular cell and then keying each transmitter up individually. An associated drive test time, in the meantime, drives the area in a motorized vehicle to observe any possible interference. This method is inherently very labor intensive and costly. In addition, the drive-test approach, while sometimes useful, does not take into account variations in mobile station types. The drive test approach is also quite expensive because it requires intensive resources. Each possible offending frequency is "keyed-up," while the remaining frequencies are turned off, in order to identify offenders.
From the foregoing, it can be appreciated that a need exists for a reliable method and system for identifying and analyzing sources of downlink interference in cellular radio networks, without the difficulties and expense associated with the methods described above. Such a method and system, if implemented, would serve to promote increased efficiency in downlink interference identification and analysis in wireless telecommunications networks, such as cellular radio networks, which in turn would promote optimization of network functions and operations.